The present invention relates to a die used for producing an optical panel having an optical pattern (e.g. dots or lines) integrally formed on at least one side of the optical panel, which is, for example, an edge light panel or a light-diffusing panel, which is used in a backlight or front light for a liquid crystal display, and various illuminators such as electric signs; a process for producing the die; and a process for producing an optical panel by using the die.
Such an optical panel having an optical pattern formed on at least one side thereof is generally produced by injection molding of a transparent thermoplastic resin. For giving such an optical pattern integrally, there have been known, for example, a method using a mold having concavities and convexities corresponding to the optical pattern of a product which are formed on the predetermined surface of the cavity by machining (e.g. cutting or blasting), electrical discharge machining, etching or the like, and the method proposed in JP-A-9-222514 in which to the inner surface of the mold of an injection molding machine is attached a nickel thin die called stamper which has concavities and convexities corresponding to the optical pattern of a product and formed on the surface of the die, and is obtained by conducting nickel plating on an uneven surface formed by the use of a photoresist and then peeling the photoresist layer.
However, in the case of the mold subjected to machining (e.g. cutting or blasting), electrical discharge machining, etching or the like, the minimum size of the concavities and convexities formed is as large as at least hundreds micrometers. Therefore, the mold is disadvantageous in that when light is introduced into an optical panel injection-molded by the use of the mold, the contrast between light and darkness is too high owing to the pattern, so that it is difficult to obtain uniform light for illumination. On the other hand, the stamper made of nickel involves, for example, the following problems: a very long process is necessary for producing such a stamper; the unit cost of the stamper is high because its production entails high cost of material and running cost and requires a long time; and moreover the production is difficult because individual steps in the production are difficult, so that each of them requires experience and know-how.
The present inventors has found a technique for forming a molding pattern of a photosensitive heat-resistant resin directly on the surface of a metal substrate by a photoresist method using the photosensitive heat-resistant resin and has proposed the technique in JP-A-2001-337229. When this method is adopted, a die having a very precise optical pattern formed thereon can be obtained at a low cost in a short time. However, experiments after the proposal elucidated the following: the release properties in molding by the use of the die are not always good, so that it is necessary, for example, to increase cooling time in a mold a little, and the unsatisfactory release properties tends to affect the durability. SUMMARY OF THE INVENTION
The present inventors further investigated in order to solve such problems, and consequently found that when the whole of the surface of the metal substrate of a die which has a molding pattern made of a photosensitive heat-resistant resin and corresponding to an optical pattern is covered with said photosensitive heat-resistant resin, the release properties of a molded optical panel from the die is improved; and that when the surface of the metal substrate as a starting material for the die is previously roughened, the release properties and the durability of the die are further improved, whereby the present invention has been accomplished.
Therefore, an object of the present invention is to provide a die obtained by improving the molding die disclosed in JP-A-2001-337229, namely, to provide a die more suitable for producing an optical panel having an optical pattern (e.g. dots or lines) integrally formed on at least one side of surfaces of the optical panel, which is, for example, an edge light panel or a light diffusion panel, which is used in a backlight or front light for a liquid crystal display, and various illuminators such as electric signs; a process for producing the die; and a process for producing an optical panel by using the die.
According to the present invention, there is provided an optical-panel molding die for integrally forming an optical pattern on at least one side of surfaces of an optical panel, the die comprising a metal substrate and a molding pattern corresponding to said optical pattern and formed of a photosensitive heat-resistant resin on one side of surfaces of the metal substrate, wherein the whole of the surface having the molding pattern of the metal substrate is covered with a layer of said heat-resistant resin. In this molding die, it is advantageous to roughen previously the surface of the metal substrate which is to be covered with said heat-resistant resin.
In addition, according to the present invention, there is also provided a process for producing the above-mentioned optical-panel molding die which comprises the steps of forming a layer of a photosensitive heat-resistant resin on the surface of a metal substrate, exposing the layer of the photosensitive heat-resistant resin to light through a mask having a pattern corresponding to the optical pattern of the optical panel, and developing the layer to form a molding pattern corresponding to the pattern of the mask on the layer of the photosensitive heat-resistant resin, wherein the photosensitive heat-resistant resin is left all over the one side of surfaces of the metal substrate also after the development.
Specifically, it is possible to adopt a method in which the development is terminated before the complete dissolution of portions to be developed of the photosensitive heat-resistant resin, whereby the photosensitive heat-resistant resin is left also on areas of the metal substrate which correspond to the developed portions of the resin. It is also possible to adopt a method in which two layers of the photosensitive heat-resistant resin are formed on the surface of the metal substrate and the first layer is deprived of its photosensitivity after its formation, whereby the first layer having no photosensitivity is left on the metal substrate.
Furthermore, according to the present invention, there is provided a process for producing an optical panel having an optical pattern integrally formed on at least one side of surfaces of the optical panel, the process comprising the steps of setting, on at least one of the two surfaces of a mold, a molding die comprising a metal substrate and a molding pattern corresponding to the optical pattern of the optical panel and formed of a photosensitive heat-resistant resin on one side of surfaces of the metal substrate, the whole of the surface having the molding pattern of the metal substrate being covered with a layer of said heat-resistant resin; and injecting a transparent molten resin into a mold cavity comprising the molding die as at least one of its surfaces, to mold the resin.